This invention relates to metal halide high intensity discharge lamps and, in particular, to a metal halide lamp operative with a constant wattage-type mercury ballast.
One such lamp is disclosed in U.S. Pat. No. 3,262,012, dated July 19, 1966, issued to F. Koury et al. This lamp comprises an envelope containing a vaporizable, positively ionizable material, at least two electrodes enclosed by the envelope between which an electric arc discharge is established, electric impedance means connected in series with one of the electrodes, and thermostatic switch means responsive to heat from the arc to shunt the impedance. With the impedance in series with an external ballast, current flow is maintained sufficiently high between peaks to prevent extinction of the arc during the period that the arc tube is warming up to operating temperature and impedance.
A metal halide lamp utilizing a voltage doubler circuit is described in U.S. Pat. No. 3,900,761, dated Aug. 19, 1975, issued to Freese et al. This lamp is described as having two main electrodes and a starter electrode, and has a resistor and a diode connected between the starter electrode and the adjacent main electrode. A second resistor is in circuit between the starter electrode and its connector to an external power supply. This circuit permits the lamp to be started on a high-pressure mercury vapor lamp ballast.
A lamp utilizing a thermally operative switch to isolate the starter electrode is disclosed in U.S. Pat. No. 3,619,710, dated Nov. 9, 1971, issued to John F. Waymouth. The Waymouth patent discloses that in an arc tube of an arc discharge lamp having two main electrodes and a starter electrode adjacent one of them, with the starter electrode being electrically connected to the adjacent electrode through a resistor external the arc tube, the starter electrode is electrically connected to the other main electrode through a thermally operative switch and a second resistor. In normal operation the switch opens after lamp ignition to isolate the starter electrode from the circuit of the other main electrode and to place the starter electrode at substantially the same potential as the adjacent main electrode to prevent electrolysis of the molybdenum ribbon leading to the starter electrode.
A lamp utilizing a fuse heater and a shunting thermal switch is disclosed in U.S. Pat. No. 4,013,919, dated Mar. 22, 1977, issued to Eugene K. Corbley. The lamp comprises an inner arc tube and an outer glass envelope. The outer glass envelope normally prevents the emission of ultraviolet radiation, but if the outer envelope is broken, such harmful radiation may be released. To prevent this, a fuse heater and shunting thermal switch is connected in series with the arc tube and located within the outer envelope. Should the outer envelope be broken, air cools the switch so that it opens. Current flow through the heater then raises its temperature and causes it to oxidize, thereby opening the circuit and disabling the lamp.